A technique for forming a transistor by using a semiconductor over a substrate having an insulating surface has attracted attention. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit and a display device. A silicon film is known as a semiconductor applicable to a transistor.
Whether an amorphous silicon film or a polycrystalline silicon film is used as a semiconductor in a transistor depends on the purpose. For example, in the case of a transistor included in a large display device, an amorphous silicon film, which can be formed using an established technique for forming a film over a large substrate, is preferably used. On the other hand, in the case of a transistor included in a high-performance display device where driver circuits are formed over the same substrate, a polycrystalline silicon film, which can form a transistor having high field-effect mobility, is preferably used. As a method for forming a polycrystalline silicon film, high-temperature heat treatment or laser light treatment which is performed on an amorphous silicon film has been known.
In recent years, an oxide semiconductor has attracted attention. For example, a transistor including an amorphous In—Ga—Zn oxide film is disclosed (see Patent Document 1). An oxide semiconductor film can be formed by a sputtering method or the like, and thus can be used for a semiconductor film of a transistor in a large display device. A transistor including an oxide semiconductor has high field-effect mobility; therefore, a high-performance display device where driver circuits are formed over the same substrate can be obtained. In addition, there is an advantage that capital investment can be reduced because part of production equipment for a transistor including an amorphous silicon film can be retrofitted and utilized.
In 1985, synthesis of an In—Ga—Zn oxide crystal was reported (see Non-Patent Document 1). Further, in 1995, it was reported that an In—Ga—Zn oxide has a homologous structure and is represented by a composition formula InGaO3(ZnO)m (m is a natural number) (see Non-Patent Document 2).
In 2012, it was reported that a transistor including a crystalline In—Ga—Zn oxide film has more excellent electric characteristics and higher reliability than a transistor including an amorphous In—Ga—Zn oxide film (see Non-Patent Document 3). Non-Patent Document 3 reports that a crystal boundary is not clearly observed in an In—Ga—Zn oxide film including a c-axis aligned crystal (CAAC).